The producers of the Hurt Locker are targeting almost 25,000 Americans in
court for allegedly pirating their film. But what is BitTorrent and what
role does it have in copyright infringement?

BitTorrent is just one of many protocols for exchanging data via the internet. Others include HTTP, the protocol behind the web, and POP3, the protocol which delivers email to many computer desktops.

It was invented and released in 2001 by Bram Cohen, an American software developer. It was designed to allow users to share any large file with each other easily and quickly.

When a user wants to download a file, they search for the relavent .torrent file on Google or one of the specialised BitTorrent indexes on the web, such as the controversial Pirate Bay.

The founders of the Pirate Bay, which overtly helps BitTorrent users find trackers for copyright films, music and software, have been prosecuted for criminal infringement, but the site is still online.

Once they have download the relevant tracker, they use a BitTorrent client programme on their computer, such as uTorrent, to begin downloading the file. The tracker is simply information about the file that allows the client programme to find other users who are sharing copies of it.

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The BitTorrent protocol breaks up each file into small chunks which are downloaded in no particular order from multiple other users simultaneously. Once a user has started downloading a file, they also typically become an uploader, sharing the chunks of the file they already have.

A group of users sharing a particular file - say, a copy of The Hurt Locker in .avi format – is known as a swarm. Thousands of users can participate in a swarm at any one time. Even after they have completed download their copy, many BitTorrent aficionados continue sharing it as part of their commitment to the "community".

Crucially, this process is completely peer-to-peer. While Napster, the early music downloading service, required a central server to coordinate sharing, BitTorrent does not. Users are connected directly to each other, which means copyright holders seeking legal redress target individual filesharers.

Of course, BitTorrent can also be used for legitimately sharing files that do not infrincge copyright such as open source programs.